Image display projectors generate display images and project them onto display screens, typically for viewing by multiple persons or viewers. In a front-projection format, for example, the projector is positioned on the same side of the display screen as the viewers. The projector directs the display image to the display screen from which the image is reflected for viewing.
FIG. 1 is a diagrammatic side view of a prior art liquid crystal display (LCD) projector 10 having a light source 12 and a reflector 14 that direct polychromatic, substantially white light through a condenser lens 16 along optical paths 18 toward a fold mirror 20.
The light passes through a collimating Fresnel lens 22 that collimates the light for delivery to a LCD 24 such as, for example, a transmissive color thin-film transistor (TFT) active matrix LCD. LCD 24 controls or modulates the substantially white light to form a multi- or full-color display object according to a video display signal delivered to LCD 24 from display control circuitry (not shown). The video display signal may be in the form of, for example, a computer display signal that is generated in real time or retrieved from a computer memory.
The light modulated by the LCD 24 propagates to a Fresnel field lens 26 that focuses the light toward an objective projection lens assembly 28, which projects the light toward a display screen 30. Proper alignment of the optical components in LCD projector 10 is necessary to obtain display images on display screen 30 with minimal aberrations or distortions. Accordingly, optical components such as LCD 24, Fresnel lenses 22 and 26, and projection lens 28 are typically mounted in fixed positions to maintain proper alignment.
A consequence of assembling the LCD projector in such a manner is that the practicable optical design constraints require that at least minimum distances be maintained between selected optical components. For example, LCD 24 with a conventional diagonal dimension of 8.4 inches (21.3 cm) would typically be used with Fresnel field lens 26 of the same diagonal dimension and a focal length of at least about 10 inches (25.4 cm), which corresponds to the minimum path length distance between Fresnel field lens 26 and projection lens assembly 28. Collimating Fresnel lens 22 typically would have a focal length of at least about 9 inches (22.9 cm) and a corresponding separation from light source 12 of about 8 inches (20.3 cm). Conventional LCD projector 10 must encompass within its housing, therefore, a fixed minimum optical path length of at least about 18 inches (45.7 cm).
In addition to the fixed minimum optical path length, conventional LCD projector 10 includes a generally planar LCD 24 that is oriented substantially perpendicular to the optical path and generates optical information to be projected with minimal aberrations or distortions. As a result, a conventional LCD projector housing typically has width and height dimensions comparable to the planar dimensions of LCD 24 and a length dimension comparable to the conventional minimum focal lengths.
A disadvantage of such a conventional LCD projector housing is that it defines a volume that may be approximated as 2.8.times. (diagonal dimension of LCD 24).sup.3, or about 1800 in.sup.3 (29,500 cm.sup.3), in the stated example, which is relatively large as an item to be carried by an individual. More specifically, LCD projectors frequently are used as portable presentation tools, and such relatively large LCD projector housings are cumbersome and difficult to handle, particularly when traveling. Moreover, the mechanical components required for such a relatively large optical instrument must be correspondingly large and increase the weight and cost of conventional LCD projectors.
In some conventional LCD projectors, the size of the housing is decreased by employing at least one fold mirror, such as fold mirror 20, that folds the optical path between fixed optically active or powered components such as LCD 24, lenses 22 and 26, and projection lens assembly 28. Such fold mirrors function to utilize more efficiently the volume within the LCD projector housing for the optical path lengths between the fixed optically active or powered components. U.S. Pat. No. 5,321,450 of Shapiro et al. shows an example of an LCD projector with multiple fold mirrors between fixed optically active or powered components. The LCD projector of Shapiro et al. includes two fold mirrors positioned between an illumination source and the LCD, two fold mirrors positioned between the LCD and a projection lens assembly, and a projection fold mirror that projects a display image toward a display screen. As with other conventional LCD projectors, all of the optical paths between the illumination source and the projection lens assembly is contained within the LCD projector housing. In this case, the volume is reduced to about 2.2.times. (diagonal dimension of LCD).sup.3, or about 1300 in.sup.3 (21,300 cm.sup.3) Moreover, each of the five fold mirrors transmits only 94 percent of incident light, resulting in a brightness reduction of 28 percent. With image brightness consistently being a limitation in LCD projector performance, such a significant loss of luminance is a major disadvantage of using large numbers of fold mirrors to fit the optical path compactly within an LCD projector housing.